1. Field of the Invention
The present invention relates generally to a wireless communications enabling system for closed environments. More specifically, the invention relates to a method and apparatus for transferring RF energy between the interior of a coaxial cable and an external antenna.
2. Discussion of the Related Art
Contemporary mobile communication receiver/transmitter units such as found in cellular telephone systems and other types of portable radio telephone systems are able to function only to the extent that the mobile units are able to send and receive radio signals to and from a base station associated with the system. In the real world environment there are impediments to normal radio communication. For example, closed environments such as tunnels, buildings and enclosed shopping malls can attenuate radio signals by as much as 50 dB for small structures and up to total cut-off in the lengthy structures used as underground throughways for trains and road vehicles. The amount of attenuation depends on circumstances, such as the shape of the tunnel or building, and the presence of obstructions like trains or trucks and cars. This attenuation makes radio wave propagation in closed environments erratic and unreliable.
Propagation of radio signals in such closed environments is normally accomplished by propagating a radio frequency signal through either a coaxial or a bifilar conductor located within the closed environment.
Other attempts to radiate radio frequency (FR) power into problematic isolated structures, i.e., closed environments, include the use of a leaky coaxial cable in the structure, and also the brute force approach of directing a large RF power level into the structure from the various repeater locations. However, such approaches have proven to be expensive, prohibitively complicated and difficult-to-impossible to upgrade.
Leaky feeder coaxial cable is commonly used as the antenna to provide portable and mobile two-way radio coverage in enclosed tunnel and tunnel-like confinements. Leaky-feeder cable is a specially designed coaxial cable with slots in the outer shielding conductor which allow a measured amount of RF power which is running through the cable to “leak” out and thus provide a controlled signal environment within a specified distance from the cable. Reciprocity, as applied to an RF signal path, accounts for this same mechanism to couple signals from transmitting devices within this same environment to the leaky feeder cable and from there to associated receiving apparatus.
Leaky feeder manufacturers specify the linear or dielectric loss per unit of length, the same as traditional coaxial cables, and the coupling loss, which is defined as the difference in the RF power level flowing in the cable at any point and the power measured by a standard receiver 20 feet (6 meters) perpendicular to that point. This coupling loss typically ranges from 60 to 80 dB, depending upon the design of the cable. Thus, there is a linear relationship (in dBm) between the power flowing in the cable and the available power to be received by the portable or mobile radio and the power available to the fixed receiving system. Once these maximum signal parameters are determined for a particular system design, the maximum amount of dielectric loss that can be tolerated, and thus the maximum cable length, can be determined.
Common design practice is to place amplifiers at regular intervals along the leaky-feeder system, located at the point in the cable where the RF power reaches the design minimum. The amplifier boosts the signal enough to make up for the dielectric loss expected in the next section of cable, thus making sure that the signal levels never drop below the design minimum. U.S. Pat. Nos. 5,603,080 and 5,404,570 for “Radio Coverage in Closed Environments” issued to S. Kallander and P. Charas are examples of repeater systems.
In many systems, due to physical or other constraints, it is not possible to replace a cable or place an amplifier at the technically required location. In such cases, the signal levels fall below the design requirements and communications is degraded and becomes unusable until the next amplifier is encountered. In public safety and other critical communications systems, areas of degraded communications are not tolerated. A simple way of enhancing this signal is desirable. An effective way is to tap into the cable and place a simple antenna at that location to effectively reduce the coupling loss of the cable at that point. When the signal level within the cable is known, the required distance between these devices can be determined to provide required coverage until the next amplifier is encountered.
In some systems, it is also required to bring coverage into areas adjacent to the leaky-feeder coverage area, but separated by distance or an intervening structure such as a wall. It is then desirable to tap into the leaky-feeder cable in some way to connect another branch feedline and antenna system to cover this adjacent area.
Prior art required cutting the cable, attaching connectors and inserting a coupling component to which an antenna or feedline would be attached. This is time-consuming and expensive and, in the case of a working system, at least part of the system would be out-of-service until the connectors could be attached to the coupling device.
Other prior art obviates the need for cutting the cable and installing connectors, but requires cutting through the leaky-feeder cable dielectric and attaching a device to the center conductor of the cable. This is undesirable for two reasons. First, it allows for the possibility of contaminating the center conductor with the environment into which it is installed. This can cause higher dielectric losses and, depending upon the method of attachment of the device to the center conductor, spurious intermodulation products to be generated. Second, any type of connection to the center conductor of the cable has the potential of causing noise and the creation of intermodulation products, which could cause system signal degradation.
In view of the preceding, due to physical constraints placed on the location of the signal boosters, and the practical limitations on the power output of the booster, it is obvious that a need exists to increase the low level of radiation in radiating coaxial cable systems when the distance between booster amplifiers along a cable is forced to be greater than the usual design parameters dictate. This results in poor or unusable signal levels along a portion of the cable system before the next booster amplifier is reached.